1. Field of the Invention
This invention relates generally to techniques of visualizing planar fields of chemical concentration and/or temperature in rapidly moving gases and more particularly to the measurement of these quantities in turbulent flow.
2. Description of the Prior Art
The instantaneous three-dimensional characterization of reacting, turbulent flows is the ultimate goal of optical diagnostics of gases. Complete understanding of this environment requires simultaneous, spatially resolved measurements of both chemical and fluid dynamic parameters. Much progress toward this goal has been made using Rayleigh and Mie scattering. However, these scattering techniques cannot differentiate chemical species.
Planar laser induced fluorescence (PLIF) is the technique that competes most directly this proposed method because it is fast, capable of high spatial resolution, and can differentiate chemical species. FIG. 1A illustrates the difference in implementation between PLIF and optical absorption tomography. PLIF operates by crossing a flow of gas 2 with a sheet of light 4 tuned to an absorption of the chemical species of interest. A fraction of the molecules are excited from the ground state to an excited state of the molecule. PLIF images the emitted light 6 with an 2D detector 8 (electronic camera) and attempts to infer the magnitude of N.sub.0 from this light. Tomography measures the amount of light absorbed across the sheet of light 4 using a one dimensional detector 10 creating a "projection." Projections must be measured at many angles around the flow and these data are used to "reconstruct" a cross sectional map of the concentration using computer algorithms.
PLIF suffers from collisional quenching that can render its measurements difficult to interpret quantitatively in practical flows. This is illustrated using FIG. 1B. Here, N.sub.0 is the number of molecules in the ground state (concentration to be measured) and N.sub.1 is the number of molecules excited by the laser radiation E. The number of molecules excited (N.sub.1) exits this upper state via two channels: collisional de-excitation (quenching) Q and fluorescence F to an intermediate state in which light is emitted. The difficulty lies in the fact that the collisional deactivation rate Q generally is not known and this rate is many times larger than the deactivation rate due to fluorescence F (except in very low pressure flows). In optical absorption tomography the absorption depends only on N.sub.0 and hence is independent of the unknown quenching rate Q.
Tomography was first suggested for the study of reacting flows by R. Goulard and P. J. Emmerman, Topics in Current Physics, 20; Inverse Scattering Problems in Optics, H. P. Baltes, ed., Springer-Verlag, New York, p. 215, (1980). However, even though optical tomography can be applied to a wide class of important fluid dynamic problems, implementation has been limited to a few proof-of-principle studies. For example, R. Goulard and S. R. Ray, Advances in Remote Sensing Retrieval Methods, A. Deepak, H. E. Fleming, and M. T. Chahine, eds., A. Deepak Publishing, Hampton, Va. (1985) and S. R. Ray and H. G. Semerjian, Paper 83-1553, AIAA 18th Thermophysics Conference, Montreal, Canada (1983) have measured temperature and OH concentration fields in a steady-state, premixed flame with a continuous wave (cw) ring dye laser. Absorption experiments used fan beam geometry and either an Ar.sup.+ laser through a rotating mirror to study an iodine plume (K. E. Bennett, G. W. Faris, and R. L. Byer, Appl. Opt. 22, 2678-2685 (1984); K. Bennett and R. L. Byer, Opt. Lett. 9, 270-272 (1984)) or a lamp source directed through a rotating chlorine jet (G. W. Faris and R. L. Byer, Opt. Lett 7, 413-415 (1986)) to create projections. More recently, R. Synder and L. Hesselink, Appl. Opt. 24, 4046-4051 (1985), demonstrated a novel configuration using holographic optical elements and a rotating mirror.
These studies share common shortcomings: they all rely on rotating elements and cw lasers, thus restricting measurements to a millisecond time scale. In order to be effective, a species-specific optical tomography instrument capable of imaging turbulent structure and temperature in fast reacting flows must collect data on a microsecond time scale.
The time resolution required of an instrument is determined by the necessity to freeze a resolution element. The size of the resolution element should approach the smallest space scale of the turbulence. For flow velocities not much greater than 100 m/s, the smallest space scale, or eddy, will be approximately 1 mm. If an element is considered to be stationary if it does not move more that 10% of its size during the measurement, all data must be collected in 10.sup.-6 s. As the velocity decreases and the scale size increases, this time increases considerably. Finally, about 10,000 pieces of data (pixels) should be collected during that time to create a meaningful image.
The primary object of this invention is therefore an absorption tomographic instrument capable of imaging one or more chemical species and/or temperature with sufficient temporal and spatial resolution to resolve turbulent structure in high speed gaseous flows. A related object of the invention is to capture nominally 100 projections of 100 elements each on a microsecond time scale.